This invention relates to microcapsules, each of which has a core particle of solid material coated by at least a thin, pinhole-free inner layer, which insulates and protects the core particle, and an outer thermoplastic layer that facilitates molding the microcapsules into articles in which the solid core particles, still separate from each other, constitute a high percentage of the mass. The invention also relates to the method of forming such microcapsules. In particular, the invention relates to microcapsules that have solid core particles of magnetic material coated first with a thin, conformal layer of parylene and then with a layer of thermoplastic material.
The microcapsules formed in accordance with this invention are called "reservoir microcapsules" in reference to the fact that each one contains a solid core particle, as distinguished from matrix, or cluster, microcapsules formed by mixing solid core particles with coating material, allowing the latter to solidify in chunks or sheets, and then grinding the chunks or sheets into microcapsule-sized pieces, each of which may contain several--or none--of the original core particles. The matrix encapsulation technique is particularly disadvantageous in the manufacture of articles that require superior magnetic qualities, whether those qualities are associated with material that retains magnetism induced in it, that is, ferromagnetically hard material, such as is used in permanent magnets, or with ferromagnetically soft material that retains little or no magnetism when the inducing field is removed.
The latter type of material is used as core material for coils, and, essentially, its purpose is to concentrate the magnetic flux associated with the flow of current in the coils. For this reason, it is desirable to have ferromagnetically soft material that has a high permeability. When the magnetic flux varies, whether due to variations in current in a coil linked with the core or because of interaction with another magnetic field, eddy currents are produced in the core. These currents cause losses of energy in the core, and such losses increase with the rate of change of the flux producing them and with the lengths of the eddy current paths. They are inversely proportional to the resistivity of the core material. If the device is to operate at its maximum efficiency, the eddy current losses must be minimized, although not to the extent of reducing the permeability of the core too much.
It is a well-known practice, at least for a coil that operates at relatively low frequencies, to make the core out of silicon steel, a material that not only has a high permeability but also has a relatively high resistivity. The core is not made of one large piece of such material but is built up out of laminations, each insulated from all adjacent laminations by a layer of insulation to constrain each eddy current path to one lamination. The eddy current losses are proportional to the cube of the thickness of the laminations and it is, therefore, very important to make the laminations thin, but not so thin that the cumulative thickness of the layers of insulation will be an appreciable fraction of the thickness of the core, displacing too much of the high-permeability ferromagnetic material and reducing its effective permeability.
In the case of a coil to be used at frequencies so high that the eddy current losses in a laminated core would be too great, it is common to make the core of powdered magnetic material mixed or coated with a thermoplastic binder and molded in the proper shape under pressure that may be on the order of 100 tons/in.sup.2 or even more. In order to keep the effective permeability of the molded core as high as possible, the magnetic material must constitute as great a percentage of the core as possible. At the same time, it is important to keep the magnetic particles insulated from each other to prevent the formation of eddy current paths that are any longer than the dimensions of a single particle.
Permanent magnets have different criteria, especially those in which rare-earth elements play an important part. While some of them have extremely large B.times.H products and would seem to be well suited for use in such things as permanent magnet motors, they are so fragile that even a slight mechanical shock can cause them to turn to fragments.
It is well known that the magnetic characteristics of many magnetic materials deteriorate if the materials are oxidized. The tiny particles that are to be encapsulated in accordance with this invention are particularly susceptible to oxidation, and it is essential to prevent that from happening. It is for that reason, as well as to keep each particle electrically insulated, that the microcapsule shell must be free of pinholes and preferably hydrophobic.